Much like the workout it documents, the book is a relentless, breathless march through CrossFit’s history, the science behind its regimen, and the men and women who live by it.

The origins of CrossFit can be traced to a former gymnast, Greg Glassman, who grew up working out on the public gymnastic rings in San Fernando Valley in the ’70s.

What drove Glassman to develop what we now call CrossFit was a desire to reach “That Feeling.” “That Feeling” was a nirvana state of gasping, near-vomiting exhaustion that ring gymnasts had to overcome to dismount with a smile. And, as Herz writes, the $19.95 Ted Williams weight-lifting set just wasn’t cutting it.

So, Glassman began to tinker around with ways to reach that masochistic bliss. In the story, he does 21 reps of bringing a weighted bar to his chest, squatting, and then exploding upwards and bringing the bar above his head (now called a thruster). Oh—and this was followed by 21 pull-ups, then 15 thrusters, 15 pull-ups, nine thrusters, and nine pull-ups.

Then he threw up.

Over the next decades, as Glassman bounced from personal training job to personal training job at various gyms, he began to develop what would become CrossFit workouts. Then, in the ’90s, he was hired to train members of the Santa Cruz Police Department, most notably the original “firebreather,” Greg Amundson, a policeman. Glassman’s classes became increasingly popular via word of mouth, and the first affiliate was opened in 2000 in Seattle.

Over the ensuing decade, largely due to its popularity among police forces, on military bases in war zones, and in online communities, the CrossFit movement grew exponentially. In the aftermath of the two wars, its popularity on bases spread to the general public when veterans returned home, through veterans groups like Red, White & Blue, which Herz is involved with.

In 2005, there were 13 affiliates (or Boxes as they are known); by 2012, there were 3,400. There are now more than 9,000 worldwide, and the extremely popular yearly CrossFit Games.

Nowadays, CrossFit is just as likely to be in the news for safety concerns as well as its fat-burning successes. There have been concerns about injuries from improper form and technique as well as placing its users at risk of rhabdomyolysis, a condition in which broken-down muscle cells are released into the bloodstream, potentially causing kidney failure.

To both of those concerns, however, Herz is dismissive in three parts.

“I don't think rhabdo is the risk to really watch out for, for the general public. If you look at the stats on rhabdo, and where there’s really a lot of it—it's high school football, football camps in particular,” she says in an interview with The Daily Beast. “So the real rhabdo is when you get people who are starting out pretty strong, they are already athletes, but they are out of shape, and then they jump into it and go for it” and do more than their body can handle. “They think they can hold their own with the firebreathers there.”

“They do have to worry about the injuries that you can trigger if you go heavy without mastering movements,” she admits. However, that is a bit misguided as something to hold against CrossFit as a whole.

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First, Herz argues, our ability to weigh risk is out of whack, because CrossFit is more like a sport than a Pilates class.

“So when people sign up for CrossFit, they have to know that, if I were going to sign up and do mountain biking, or rock climbing or gymnastics, I would focus on how to do things right, because everyone knows and understands that if you do the wrong thing on a beam it's going to hurt,” she points out. Our willingness to learn the proper forms, as well as our appetite for risk when it comes to injury, are both higher when it comes to sports than general fitness, and this is how CrossFit should be approached.

“Like any sport, it’s going to involve a high level risk than being on the elliptical,” she says laughing.

Second, those who are getting injured by doing the exercises improperly often fall into one of three camps. Those who are beginners and did not look for a Box where there was an on-ramp for beginners, people (mostly men) trying to compete with CrossFit members who have been doing it for much longer, and those who attempt tougher exercises at home. The injuries, she says, “Are driven by ego and lack of judgment.”

Herz herself is no shrinking violet. In the ’90s, she was The New York Times’ first video game critic. Then after 9/11 and the Dot Com Crash, the money for a lot of the tech consulting she was doing in New York City dried up. And so, people she worked with the military had her come in.

“I ended up working in national security for what turned out to be eight or 10 years. I kind of vanished off the face of the literary map, and ended up working in the Pentagon and DARPA and other places,” she says. Then, as she got sucked into the world of CrossFit through her husband, who joined while recovering from an injury, she felt compelled to write a book about a movement she saw captivating so many people.

The first rule of CrossFit is talk a lot about CrossFit.

Anybody who’s ever known a person who does CrossFit can confirm that the participants won’t shut up about it, leading many to ascribe a cultish vibe to the enterprise.

“I think if you look at the signature elements of a cult…I don’t think you find a lot of them in CrossFit,” responds Herz. Instead, she likes to compare it to both parenthood and the intensity of team superfans.

“People have babies, and that’s all they can talk about—their baby gear, and everything. It's something they’re passionate about, it’s something that transforms their identity, and they’re just not going to shut up about it,” she explains.

Likewise the super fan of a major sports team: “The people looking at it from outside wonder what the heck is making that guy paint his face and scream his head off from the cheap seats—but there are some very primal reasons for it. And CrossFit is similar, in that the primal experience can weird people out.”

Herz’s book is a bit guilty of that rabidity—I certainly had trouble getting excited about the multiple accounts of the CrossFit Games—but perhaps it’s because in the stories she uncovers, it truly changed lives.

One of those powerful stories is of Christmas Abbot, Ray Biley, and Chazz Rudolph. Christmas worked in laundry for a defense contractor on a military base in Iraq. Ray and Chazz were both former Marines employed by Blackwater during Iraq War, where they worked on the American ambassador’s protection detail. The unlikely trio became inseparable, and pushed each other to insane levels of exhaustion and physical fitness.

“I found myself listening to Ray Biley talk on the phone, and the things he said were just so arresting and they really stayed with me,” says Herz. “He was reflecting on his life and his physical accomplishments and how he couldn’t quite do those things that he used to do, but also how it felt to be in that position, and the loss of mindfulness that you can sometimes experience when life just goes on.”

Once he started doing CrossFit, he told her, “It woke something up inside me.”

“It was a personal transformation. It was about being more alert and awake and in the moment,” she continues. “The fact that doing this training triggered some personal transformation for him was arresting and profound and worth thinking about. This wasn’t a guy out there in Barnes & Noble picking up self-help books. This guy was a personal security contractor.”

The second powerful story is the backstory to the now famous “Nasty Girls” video of CrossFit competitors Eva T., Annie Sakamoto, and Nicole Carroll.

The video unfolds over 13 minutes as the women plow through a workout: three rounds of 50 squats, seven muscle-ups, and 10 hang power cleans (bringing weighted bar from hips to rest on the shoulders). As the video progresses, Nicole falls behind, working visibly harder in later rounds to complete the tasks. And then, in an emotionally tense few seconds, sobbing, she completes the final hang power clean, barely standing upright.

What happened next demonstrated the power of the CrossFit community. When the video was posted on CrossFit HQ’s website, the site was immediately flooded with pages of comments from guys documenting that they took even longer time than she had and had to substitute easier exercises. Then there were comments such as, “Nobody in my gym, nobody, pushes themselves that hard. Inspiring.” And “I was cheering out loud for Nicole at the end! (anyone else do that?)”

“I visited a lot of CrossFit gyms, and generally, I think it's about, my estimate, and this is not scientific, is 60-40 male to female,” says Herz.

The female segment of the CrossFit community is somewhat surprising.

“You have barbells, you have intensity, and you have the exercise and expression of physical power, which is what makes you think it’s a male thing,” explains Herz. “There’s not a lot out there in fitness-land for women that involves physical power, and in fact it's almost taboo.”

CrossFit is in some ways more transformative for women than men.

“If you go to most gyms, there are mirrors everywhere. Most women’s fitness goals have to do with how they look in the mirror. And it makes people neurotic, to fixate on how their thighs look,” she reasons.

Instead, with CrossFit, the fitness is about function.

“It changes how you think about yourself. It’s really rewarding to get a pull-up. When you can pull your own body up, it’s empowering,” remarks Herz. “I can lift things, and I don’t have to ask for help when I need to move something heavy from point A to point B. The separation of physical goals from the cosmetic is embraced wholeheartedly by the women who do CrossFit.

“The attention goes away from looking at myself in the mirror. Looking at other women in the mirror. Comparing myself to other women. Knowing that they’re comparing themselves to me, to OK, can I deadlift my body weight?”

To step into those adrenaline-heavy workout zones can be a bit daunting. So to the guys who are on the edge of considering it, Herz has one bit of advice.

“If you do CrossFit for six months, you’ll be sore all the time, but you will find that you are physically stronger than you ever have been.”

As for the ladies out there—while Herz recommends her single guy friends join yoga to find the single women, “CrossFit is where the boys are!”

“If I were a single girl right now, this is a very target-rich environment. Not only are these guys in shape, but they’re not intimidated by strong women, which was a huge hurdle when I was dating.”

And no, ladies, there is no maximum number of gyms you can sign up for.